Don't return response.OutputStream if response is null

Throw an 'InvalidOperationException' instead

Change-Id: If8830abc67849f61f60320f32c3c51ccf9e8fc6e
Reviewed-on: http://review.couchbase.org/27358
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Saakshi Manocha <saakshi.manocha@globallogic.com>
Tested-by: Saakshi Manocha <saakshi.manocha@globallogic.com>